In the last decade antennas constructed using printed circuit techniques have become very popular, especially for mobile applications. These antennas are often very thin and can be affixed to a vehicle, aircraft, etc. without appreciably altering the host structure. Since they do not protrude substantially from the surface upon which they are mounted, they cause little aerodynamic drag and have low susceptibility to mechanical damage. Also, they are generally economical to construct and light in weight. The antenna of this invention retains most of the advantages of planar antennas as outlined above, but greatly extends the operating capability at a cost of modest added complexity.
Many planar antennas of the prior art can be classified as high-Q resonant devices. The conventional microstrip patch antenna is in this category. Since the input impedance of such an antenna varies rapidly with a change of frequency in the vicinity of resonance, its operating bandwidth is severly limited, typically only a few percent.
Various designs have been proposed to overcome this problem. The combining of two elements that have complementary impedances has been successfully employed to produce near-constant impedance over a very wide band. See, for example, U.S. Pat. No. 3,710,340 which was issued to an inventor hereof on Jan. 9, 1973. In that invention a monopole and a cavity-backed slot were fed at the same position on a transmission line that continued past the two radiators and was then terminated at an arbitrary point with an impedance that was equal to the characteristic impedance of the line. The two radiators, the monopole and the slot, presented different impedance characteristics to the feeder. The monopole presented a shunt impedance which approached infinity as the frequency decreased. The slot presented a series impedance that approached zero as frequency decreased. By proper design these impedances were made very nearly complementary to one another.
In co-pending U.S. Pat. application Ser. No. 906,852 now U.S. Pat. No. 4,823,145 to Mayes and Tanner, another design is shown wherein the desired impedance characteristic is achieved by shaping the ground surface such that the ratio of the width of the radiating element to its distance from the ground surface stays constant for a given curvature.